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6.1.1 Solid/Porous Fiber Optic

Use: Detecting the presence of total petroleum hydrocarbons (TPHs) in water or vapor phase. A variety of chemical sensors using fiber optic technology are in developmental stages. When combined with the cone penetrometer, fiber optic chemical sensors are easily placed in monitoring wells or soil.


Fiber optics is a technique that transmits light through long, thin, flexible fibers of glass, plastic, or other transparent material. Parallel fibers bundled together can be used to transmit complete images. The most common fiber-optic sensors send an excitation signal from a light source that is transmitted down the cable to a sensor. The sensor fluoresces and provides a constant-intensity light source that is transmitted back up the cable and detected as the return signal. The intensity of the return signal is reduced if the target contaminant is present (the intensity of the light that is recorded by the detector is inversely proportional to the concentration).

The configuration of a fiber-optic sensor system requires a simple light source, a detector, and simple optics to focus and guide light into and out of the fiber-optic conduit. The same fiber can be used to transmit the probe beam to the sensor, as well as to carry the modulated signal back to the detector. At the proximal end of the fiber is a small calculator-size box of optics and electronics that contains both the light source and signal detection equipment (generally the fiber optic cable is attached to a spectrophotometer or a fluorometer which contains both a light source and a detector). The electronics box is configured to a small central processing unit (CPU) or a lap-top computer that collects and analyzes the sensor signals and provides useful information on the analyte concentration. At the distal and working end of the fiber is the sensor, normally encased in a protective metal shield to prevent damage.


11. TPHs


Soil/Sediment Water Gas/Air
Requires extraction to liquid or gas phase BETTER ADEQUATE
Selectivity: Technique measures the contaminant indirectly.
Susceptibility to Interference: Low.
Detection Limits : 10-100 ppm (soil); 0.5-10 ppm (water).
Turnaround Time per Sample: Minutes.

Applicable To:

Screen/Identify Characterize Concentration/Extent Cleanup Performance Long-Term Monitoring
Quantitative Data Capability: Data become quantitative with additional effort.
Technology Status: Commercially available technology with limited field experience.
Certification/Verification: Technology has not participated in CalEPA certification and/or CSCT verification program.
Relative Cost per Analysis: Least expensive. Because sensors can be placed in small boreholes (0.5-inch diameter), drilling and monitoring well installation costs could be reduced. Considered to be low maintenance.


ASTM Standards/EPA Methods:

No applicable ASTM standards or EPA methods are cited for this technology.

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